Program Director/Principal Investigator (Last, First, Middle):GALAN, JORGE E 1 R01 AI 070949 - 01 A2 Salmonella enterica serovar Typhi (S. Typhi), the cause of typhoid fever in humans, continues to be a very significant health problem. lt is estimated that there are 16,000,000 cases of typhoid fever every year, resulting in 600,000 deaths. Unlike other Salmonella enterica serovars, which can infect a variety of hosts, S. Typhi is an exclusive human pathogen. The molecular bases for the host adaptation and unique pathogenicity of these bacteria are not known. However, it is believed that a combination of genome degradation and acquisition of new genetic information has conferred on S. typhiits unique pathogenic properties. The availability of the nucleotide sequence of different S. typhiisolates has provided unique insight into its potential determinants of pathogenicity. Recent work in our laboratory has focused on a "pathogenicity islet" that is unique to S. Typhi. This islet encodes novel toxin that we have named "typhoid toxin". Thisioxin is composed of two active subunits, CdtB, a homolog the active subunit of the Cytolethal Distending Toxin (CDT), and pltA, a homolog of the active subunit of Pertussis toxin. We have found that the expression of this ioxin occurs exclusively when S. Typhi has reached a specific intracellular compartment within host cells. Recent work in our laboratory has also identified a regulatory protein of previously unknown function, STy044g, which specifically controls the intracellular expression of this toxin. We have found that STY044B, which we have named lgeR (for intracellular gene expression regulator), exerts its function as a repressor by binding directly to the toxin promoter. We have also found that constitutive expression of igeR in S. Typhimurium resulted in a drastic reduction in its mouse virulence. We therefore believe that the characterization of the lgeR regulon and its mechanism of action would provide a unique insight into the intracellular biology of this ver/ elusive bacterial pathogen, as well as provide information into the biology of intracellular pathogens in general. lt is therefore the purpose of this project to define the lgeR regulon, to investigate its mode of action and to examine its contribution to virulence using a variety of in-vitro assays and a novel animal model of infection.